A continuously variable transmission is a transmission which is capable of varying a speed ratio continuously.
In a vehicle installed with a continuously variable transmission, an internal combustion engine can be operated more efficiently than in a vehicle having a conventional stepped transmission, and therefore improvements in a power performance and a fuel performance of the vehicle can be expected.
To improve the power performance and fuel efficiency performance of a vehicle installed with a continuously variable transmission even further, a possible speed ratio range (to be referred to hereafter as a “ratio range”) of the continuously variable transmission is preferably enlarged.
When the ratio range of the continuously variable transmission is enlarged, a speed ratio further toward a Low side is used during startup and acceleration, enabling an improvement in the power performance of the vehicle, and a speed ratio further toward a High side is used during high-speed travel, enabling an improvement in the fuel efficiency performance of the vehicle.
To enlarge the ratio range of a belt continuously variable transmission, a diameter of pulleys around which the belt is wound may be increased. However, an increase in pulley diameter inevitably leads to an increase in the size of the continuously variable transmission.
JP2000-346169A, published by the Japan Patent Office in 2000, proposes that a subtransmission mechanism having two forward speeds be provided in series with the continuously variable transmission either between the internal combustion engine and the continuously variable transmission or between the continuously variable transmission and a final gear. According to this prior art, by modifying a gear position of the subtransmission mechanism in accordance with operating conditions of the vehicle, the ratio range can be enlarged without increasing the size of the continuously variable transmission.